Various electro-optical systems have been developed for reading optical indicia, such as bar codes. A bar code is a coded pattern of graphical indicia comprised of a series of bars and spaces of varying widths, the bars and spaces having differing light reflecting characteristics. Some of the more popular bar code symbologies include: Universal Product Code (UPC), typically used in retail stores sales; Data Matrix, typically used for labeling small electronic products; Code 39, primarily used in inventory tracking; and Postnet, which is used for encoding zip codes for U.S. mail. Bar codes may be one dimensional (1D), i.e., a single row of graphical indicia such as a UPC bar code or two dimensional (2D), i.e., multiple rows of graphical indicia comprising a single bar code, such as Data Matrix which comprising multiple rows and columns of black and white square modules arranged in a square or rectangular pattern.
Systems that read bar codes (bar code readers) electro-optically transform the graphic indicia into electrical signals, which are decoded into alphanumerical characters that are intended to be descriptive of the article or some characteristic thereof. The characters are then typically represented in digital form and utilized as an input to a data processing system for various end-user applications such as point-of-sale processing, inventory control and the like.
Bar code readers that read and decode bar codes employing imaging systems are typically referred to as imaging-based bar code readers or bar code scanners. Imaging systems include charge coupled device (CCD) arrays, complementary metal oxide semiconductor (CMOS) arrays, or other imaging sensor arrays having a plurality of photosensitive elements (photosensors) defining image pixels. An illumination apparatus or system comprising light emitting diodes (LEDs) or other light source directs illumination toward a target object, e.g., a target bar code. Light reflected from the target bar code is focused through a system of one or more lens of the imaging system onto the sensor array. Thus, the target bar code within a field of view (FV) of the imaging lens system is focused on the sensor array.
Periodically, the pixels of the sensor array are sequentially read out generating an analog signal representative of a captured image frame. The analog signal is amplified by a gain factor and the amplified analog signal is digitized by an analog-to-digital converter. Decoding circuitry of the imaging system processes the digitized signals representative of the captured image frame and attempts to decode the imaged bar code.
As mentioned above, imaging-based bar code readers typically employ an imaging lens assembly for focusing scattered/reflected light from an object of interest within the field of view (FV) onto the sensor array. If a target object is within the field of view FV, an image of the target object will be focused onto the sensor array.
There are typically two types of imaging lens assemblies: 1) fixed focus lens systems; and 2) variable focus lens systems. In a fixed focus system, the field of view (FV) and a working range (WR) of the imaging system is fixed. The working range (WR) of an imaging system is a distance range in front of or forward of the imaging lens assembly within which a target object of interest, such as a target bar code, may be successfully imaged and decoded by the imaging system decoding circuitry.
The working range (WR) and field of view (FV) require a user to move the bar code reader relative to the target bar code such that the target bar code is within the field of view (FV) and within the working range (WR) of the imaging system for successful decoding of the imaged target bar code. For example, if the target bar code is positioned at a distance that is greater than the working range, the size of the imaged target bar code will be too small and out of focus to be well resolved by the imaging system and therefore to be success fully decoded. That is, the pixels per module (PPM) will be below a threshold value and, therefore, too small to permit successful decoding. PPM is a measure of how many active pixels of a sensor array the smallest feature (bar or stripe) of a target bar code is imaged onto. Additionally, at the near and far limits of the working range (WR), there is a problem with blurriness, that is, poor resolution of the imaged target bar code.
One type of variable focus lens system is an autofocus system, that is, one in which the entire imaging lens assembly moves with respect to a fixed sensor array. This is a fixed focal length lens assembly in which movement of the lens assembly with respect to the sensor array provides for focusing, that is, a sharp image being focused onto the sensor array. Such an autofocus system will permit, for example, sharp focusing of an image of a target bar code at the extremities of the working range (WR).
However, such an autofocus system, while addressing the problem of blurriness at the extremities of the working range, does not address the limitation of the imaging system regarding the PPM threshold value. At distances greater than the far distance of the fixed working range (WR), the PPM value is below the threshold PPM value and the imaged target bar code cannot be decoded regardless of how well the image is focused on the sensor array.
One potential solution to this problem would be to use a particular type of variable focus lens system used in photographic applications and called a zoom lens system. A zoom lens system permits changing of the focal length of the system. This could allow an effective working range (WR) of the imaging system to be increased by changing the focal length of the imaging lens assembly.
In a zoom lens system, typically there are two moving lenses, each of which move independently with respect to one or more stationary lenses. Advantageously, the independent movement of the two lenses allows the focal length (or magnification), as well as the field of view (FV) to be changed.
However, the problem with typical zoom lens systems is that they require, at a minimum, two motors: one motor for the zoom, that is, one motor to drive movement of the movable lenses that effect a change of the focal length; and a second motor for focusing, that is, keeping a sharp image focused onto the sensor array as the focal length is changed.
When the focal length changes, the location of the plane where the sharpest image is projected by the imaging lens assembly (the image plane), also changes. Thus, if applied to a bar code reader, there would need to be compensation such that the image plane remains congruent or aligned with the position of the sensor array so that a sharp image of the target bar code is focused on the sensor array surface as the focal length is changed. One way to provide for such focusing is to move the entire lens assembly with respect to the sensor array. Hence, one (or more) motors would be needed for movement of the movable lenses to effect change of focal length, while a second motor would be needed for focusing to keep the image plane aligned with the sensor array. The use of two or more motors creates complexity in the imaging lens system and may require additional time to adjust the position of both lens groups (movable lens assembly and entire lens assembly) to yield a desired image quality for successful decoding.
Since imaging-based bar code readers are typically housed in small housings decreasing the size and complexity of the imaging lens system is desirable. Further since additional drive motors increase the cost of the imaging lens system, decreasing the number of drive motors required for the imaging lens assembly is advantageous.
What is needed is an imaging lens system for an imaging-based bar code reader that has the advantages of a zoom lens system, namely variable focal length and field of view (FV), while being less complex and requiring fewer drive motors than typical photographic zoom lens systems.